1. Field of the Invention
This invention relates to an information recording and/or reproducing apparatus and more particularly to an apparatus for recording and/or reproducing information signals with a pair of heads respectively shifter by a pair of shifting elements having residual displacement characteristics.
2. Description of the Prior Art
It is recent conventional practice in the field of magnetic recording to have magnetic recording or reproducing heads mounted on electric-to-mechanical conversion elements. The heads are made to record or reproduce a signal at desired points on a recording-bearing medium by driving electric-to-mechanical conversion elements so they shift the positions of the magnetic heads. Electric-to-mechanical conversion elements usable for this purpose include, for example, a bimorph-type piezo-electric ceramic element (hereinafter called a bimorph element). An element of this kind generally has a residual displacement characteristic.
FIG. 1 of the accompanying drawings shows the voltage impressed on a bimorph element and the displacement characteristic of the element in relation to the impressed voltage. As shown, even when a voltage which is the same as previously-impressed voltage is impressed on the bimorph element, the displacement of the element is different from the previous displacement due to the previous characteristic movement of the element. Such a residual displacement characteristic has caused problems in magnetic recording and/or reproducing apparatuses with an electric-to-mechanical conversion element such as a bimorph element.
FIGS. 2A-2C show th problems of these conventional apparatuses. In FIG. 2A, the apparatus includes a base plate 1, on which bimorph elements 2 and 3 are mounted, and magnetic heads 4 and 5 mounted on the bimorph elements 2 and 3. Displacement of the bimorph elements 2 and 3 shifts the magnetic heads 4 and 5 in the directions of arrows 6 and 7, respectively. Where the displacements of the bimorph elements 2 and 3 are equal to each other, heads 4 and 5 are in alignment with each other in the direction of arrow 11, which is perpendicular to the direction of displacement of the bimorph elements 2 and 3. For an apparatus arranged in this manner, let us assume that the magnetic heads 4 and 5 are repsectively shifted to an extent la by impressing voltages of opposite directions on the bimorph elements 2 and 3. The condition schematically shown in FIG. 2B results. In FIG. 2B, reference numerals 8 and 9 denote the tracing loci of heads 4 and 5 obtained when the record bearing medium is moved in the direction of arrow 11 under the above condition. In other words, recording tracks represented by the loci 8 and 9 are obtained if a recording operation is performed under the above condition. Where the same track is to be traced by both of the two heads, the position of either the base plate 1 or the record bearing medium need be adjusted in the direction of arrow 12, with the displacements of the bimorph elements arranged to be equal to each other. However, even when the voltage impressed on the two bimorph elements 2 and 3 in the condition of FIG. 2B are changed to zero, the displacement characteristic shown in FIG. 1 prevents these elements from coming back to their initial positions and causes element 2 to be shifted from a point A to a point A' and the other element 3 to be shifted from a point B to a point B'. The residual displacement characteristic thus causes the respective bimorph elements to deviate as much as lb in opposite directions from their initial positions, which are on a straight line 10 of FIG. 2B. FIG. 2C shows this deviation.
To remove the adverse effect of the above-stated residual displacement characteristic, attempts have been made to bring the position of the bimorph element to point 0 shown in FIG. 1 by applying an alternating voltage which gradually attenuates. However, this method necessitates provision of a gradually attenuating voltage and results in a complex circuit arrangement. Besides, this method takes an excessively long period of time to remove the adverse effect of the residual displacement characteristic. More specifically, a given length of time is necessary for equalizing the displacements of the two bimorph elements from the condition shown in FIG. 2B. This method is hardly effective for instantly equalizing the extents of displacement of the bimorph elements or those of the heads.
The residual displacement characteristic also presents problems in a magnetic video tape recording and/or reproducing apparatus of the rotating two-head helical scanning type (hereinafter called VTR). FIGS. 3A-3C show the problem caused a conventional VTR by the residual displacement characteristic of the electric-to-mechanical elements. FIG. 3A shows the structural arrangement of a rotary head drum of a VTR. The VTR is provided with a rotary drum 21; bimorph elements 22 and 23; and heads 24 and 25 mounted on the bimorph elements 22 and 23. Arrows 26 and 27 indicate the directions of shift of the heads 24 and 25. The heads 24 and 25 are arranged to rotate on the same plane if the extents of displacement of the bimorph elements 22 and 23 are equal. Further, the heads 24 and 25 have the same magnetization direction.
In FIG. 3B, arrow 33 indicates the travel direction of a magnetic tape 28. In normal recording, tape 28 travels at a predetermined speed in the direction of arrow 33. Recording tracks are formed on the tape 28 one after another by causing magnetic heads 24 and 25 to trace the tape. The center lines of the recording tracks are indicated by full lines 29. When the tape is brought to a stop, the direction of the tracing loci of the heads 24 and 25 is as shown by a one-dot-chain line 30. When reproducing at a tape speed four times as fast as the normal recording tape speed, the direction of the tracing loci of the heads 24 and 25 becomes that shown by a twodot-chain line 31.
During so-called high speed search or still picture reproduction carried out at a tape travel speed different from normal reproduction speed, a noiseless reproduced picture is obtained with the heads shifted in the direction of arrow 32 by controlling the bimorph elements. FIG. 3C is a timing chart showing the timing of impression voltages impressed on the bimorph elements 22 and 23 during a four-times increased-speed searching reproduction. In FIG. 3C, wave form (a) represents a signal related to the rotation of the drum 21. The magnetic tape is traced by the head 24 while this signal is at a low level, i.e., during periods indicated by reference symbol A in FIG. 3C, and is traced by another head 25 while this signal is at a high level, i.e., during periods indicated by symbol B. Further, in FIG. 3C, wave form (b) represents a voltage to be impressed on the bimorph element 22 for shifting the head 24; and a wave form (c) a voltage to be impressed on the other bimorph element 23 for shifting the other head 25. In these wave forms (b) and (c), downslope parts on the right-hand side represent voltage portions controlling the heads 24 and 25 while they are tracing the magnetic tape 28.
When an instruction is given at a time t1 of FIG. 3C, to bring the four-times increased speed search back to normal reproduction, for example, the extents of displacement of the bimorph elements 22 and 23 should be adjusted to equal each other to equalize the positions of the heads 24 and 25 in the directions of arrows 26 and 27 (hereinafter called the head height). In such an instance, however, if the two impressed voltages are adjusted to zero at ensuing zero points or at the point in time t1, the bimorph element 22 changes its position from the point A to another point A' and element 23 from the point B to another point B', as shown in FIG. 1. This hinders adjustment of the head height and results in the deviation 2lb as shown in FIG. 1. The conventional method for preventing this deviation by using an alternating attenuation signal as the impression voltage not only results in a complex circuit arrangement but also takes an excessively long period of time to remove the adverse effect of the residual displacement characteristic in question. In other words, bringing the two bimorph elements from the points A and B in FIG. 1 to the point 0 of FIG. 1 by the conventional method, requires a certain length of time. It has thus been impossible to instantaneously equalize the extents of displacement of the bimorph elements, i.e., the shifting extents of the heads. For example, in the case of FIG. 2A, the heads remain entirely useless while the operating mode of the apparatus is switched over from high speed searching reproduction to normal reproduction. Therefore, in that instance, no satisfactory reproduced picture is obtained.